add missing require

src/routes/backups.js

@@ -8,6 +8,7 @@ exports = module.exports = {
 };
 
 var backups = require('../backups.js'),
+    BackupsError = require('../backups.js').BackupsError,
     cloudron = require('../cloudron.js'),
     CloudronError = require('../cloudron.js').CloudronError,
     debug = require('debug')('box:routes/backups'),